The present invention relates generally to medical devices for delivering cardiac arrhythmia therapy, and more particularly to an implantable device that provides a primary means for defibrillating a patient's heart and also provides a simple secondary means for defibrillation in the event of a failure of the primary means.
Pathologic ventricular tachycardia is characterized by rapid contractions of the heart's main pumping chambers, attributable to cardiovascular disorder. As a consequence of the rapid synchronous contractions, cardiac output is reduced because the ventricles are only partially filled with each heartbeat, resulting in delivery of a lowered supply of oxygenated blood to the extremities. If the ventricular tachycardia accelerates into ventricular fibrillation, the contractions of the individual cells of myocardial tissue become uncoordinated rather than synchronous with the result that cardiac output drops precipitously, and death will soon follow, absent prompt delivery of defibrillation therapy.
Typically, an antitachycardia pacemaker is implanted in the body of the afflicted cardiac patient and programmed to overstimulate the heart by delivering relatively low energy pulses at a constant rate or at varying rapid rates to suppress premature ventricular contractions. Cardioversion therapy involves delivery of electrical shocks of somewhat higher energy than the usual antitachy pulses to the heart to break the tachycardia. These therapies can lead to acceleration of a tachycardia into ventricular fibrillation in certain circumstances, so it is desirable that a defibrillating therapy be made available in the implanted device. To defibrillate the heart, one or more electrical shocks are delivered to the appropriate cardiac mass with sufficiently high energy level in an effort to halt the randomness and restore coordinated contractions of the cardiac tissue of the mass of cardiac tissue.
Multi-function automatic implantable devices may deliver a combination of therapies depending on the needs of the patient, particularly patients who are at high risk of ventricular fibrillation. Thus, it is usually proposed that automatic implantable defibrillators should be capable of performing not only defibrillation, but bradycardia and antitachycardia pacing, and cardioversion, as well. A conventional programmable pulse generator may be used for the pacing functions, with the addition of one or more relatively high capacity output storage capacitors and associated switching circuits for the charging and output functions required to produce high and relatively lower energy pulse shocks.
The appropriate therapy is to be delivered by the implanted device in response to sensing of physiologic functions or parameters of the cardiovascular system, sometimes in conjunction with other indicia of cardiac rate disorder, by means of one or more selectively positioned sensors. Typically, continuous monitoring of electrical signals indicative of the patient's cardiac activity is performed to verify that the therapy is having the desired effect, and, if it is not, to cause the delivery of additional therapy which may be more aggressive than that previously delivered, within the capability of the implanted device.
In general, while proposals have been made to provide implantable medical devices having the capability to perform a variety of different cardiac therapeutic functions, little or no attention has been given to the matter of potential device failure apart from the use of battery monitors, recommended periodic visits by the patient to the attending physician for device checks, and the like.
It is a principal object of the present invention to provide improved implantable medical devices in which device failure that would otherwise preclude defibrillation therapy is detected and corrected within the device itself, without need for surgical removal or replacement of the device.